It looks like a resistance band...and a somewhat expensive one at that. I'll bet that if you go to a place like Target, you could find a set of three bands for that price. Apparently they're pretty good for any sort of resistance training up to a point. I use one for rotator cuff exercises, but you can do just about any weight lifting exercise with them. If you search online a bit I'm sure you could find all kinds of exercises and workout routines using just a set or two of those bands.

I just bought the Champion branded 3 band resistance band package for $14.99 at target.

I had rotator cuff related problems (shoulder impingement) last summer and the PT prescribed external and internal rotation exercises with the bands to sure up the rotator cuff.

Building the rotator cuff won't neccesarily help you throw farther or better but it's important in injury prevention. With band work/rotator cuff work you want to do high reps (20-25) do them slowly and hold for one second at furthest stretch point.

If you want a bigger arm work on speed, and your legs/core. Calfs, quads, hamstrings, glutes(ass), and obliques. If you have back problems though, make sure to strengthen your shoulders, and lower back.